Chapter 12, Problem 1QC

Summary Introduction

To determine:

The product and density of DNA after one and two rounds of replication if Meselson and Stahl had started their experiment with ¹⁴N light DNA and then transferred them into ¹⁵N heavy DNA medium.

Concept introduction:

Meselson and Stahl performed an experiment to prove that DNA (deoxyribonucleic acid) replicates in a semi-conservative manner. They first grew bacterial cells in ¹⁵N medium, so that the DNA of the bacterial cells would only contain pure ¹⁵N DNA. Afterwards, these cells were transferred into ¹⁴N medium.

After one round of replication, DNA was centrifuged to observe their density in a solution containing cesium chloride. The density of parental DNA (¹⁵N/¹⁵N) was found to be $1.722$ g/cm³ and a single band was formed. The density of the DNA after replication was found to be $1.715$ g/cm³, which is the expected density of duplex containing one heavy and one light strand. Thus, they proved that DNA replicates in a semi-conservative manner.

Explanation of Solution

If Meselson and Stahl would have performed their experiment the other way around, the observations would have been different, but would definitely suggest the same thing that DNA replication takes place in a semi-conservative manner. If they would have first grown bacterial cells in ¹⁴N medium and then transferred these cells into ¹⁵N medium, DNA hybrids containing ¹⁴N/¹⁵N would have been formed after one round of replication. Their density is supposed to be $1.715$ g/cm³. After the second round of replication, half DNA molecules would be ¹⁴N/¹⁵N having the same density and the other half would be ¹⁵N/¹⁵N having $1.722$ g/cm³ density.

Conclusion

DNA replication is the process of the duplication of DNA. This process takes place in a semi-conservative manner, where one of the strands is parental and the other is newly synthesized.